1. Field of the Invention
The present invention relates to short pulse laser systems, and more specifically, it relates to the production of short-pulse, high repetition-rate, high energy output pulses.
2. Description of Related Art
Very-short-pulse laser sources (in the range of picoseconds to femtoseconds), with high repetition rates (MHz to GHz) are required for many applications, including materials processing, 3-D lithography, high-data-rate laser communication, remote sensor systems, as pump sources for the realization of short-wavelength high-energy photon sources via higher-order nonlinear optical parametric interactions, and as photo-cathode illumination laser pulses for creation of photo-electrons in high frequency particle accelerators.
For certain applications there is a long felt, unmet need for a reliable and robust source of picosecond pulses at repetition rates of 10 GHz or greater driven at the exact frequency of a desired clock. A need exists for the ability to feed bursts of electrons into every cycle of an electron accelerator and thus increase brightness. It has also been desirable for the ability to feed an etalon to create super-pulses by stacking many micro-pulses. In the latter example, a 10 GHz repetition rate would enable the use of a 15 mm etalon, which is much more convenient and inherently more stable and robust than the 1.5 m etalon required for a 100 MHz source.
Continuous wave (CW) lasers have been converted to sub-ps, high frequency pulse trains, through the use of “time-lens” techniques to generate ps-level bandwidths, followed by soliton compression at 1550 nm in specially optimized fibers to generate further bandwidth while simultaneously compressing the pulse. As is known in the art, this technical approach is limited to systems at operating wavelengths whose fiber dispersion characteristic is compatible with the requirements of soliton compression, which is satisfied at 1550 nm for available fiber materials. However, at other useful operating wavelengths, say in the range of 1050 nm, the soliton compression scheme is not feasible because the dispersion in standard fibers has the opposite sign from the dispersion at 1550 nm.
The prior art also includes controllable fs pulse-train generation techniques at 1552 nm. This approach involves optical comb signal generation using overdriven RF modulation of a cw laser diode (using a Mach-Zehnder modulator), resulting in highly chirped output pulses, followed by fiber-based frequency-chirp compensation (resulting in a picosecond pulse train), which, in turn, is followed by fiber-based dispersion-flattening that compresses the ps pulse train into fs pulses. The spectral spacing is determined by the RF modulation drive frequency, while the modulation drive power determines the bandwidth of the spectrum. Under the proper modulation conditions, a parabolic dependence of each comb mode in the ensemble can be realized, which is amenable to standard single-mode fiber compensation techniques.
The prior art also includes many examples of optical fiber-based mode-locked oscillators. See, e.g., M. Fermann, M. Andrejco, Y. Silberberg, and M. Stock, “Passive mode locking by using nonlinear polarization evolution in a polarization-maintaining erbium-doped fiber,” Opt. Lett. 18, 894-896 (1993). Such oscillators can produce pulses that, after passing through a following fiber-based or grating-based compressor, can have sub-picosecond durations. The pulse repetition rate of such oscillators is generally limited however to less than 100 Mhz, since higher repetition rates would require lengths of optical fiber that are impractically short. Moreover, such oscillators are inherently sensitive to vibration, and cannot be made as reliable and robust as can the pulse source described here.
The prior art also includes examples where the frequency of sources of sub-picosecond pulses having modest repetition rates are locked to a sub-multiple of some desired higher frequency clock; for example, a 100 MHz source might be locked to the 100th sub-multiple of a 10 GHz clock. Such sub-multiple locking schemes are difficult to implement, however, and are prone to drift and noise-related imperfections.
The ability to programmably modify the temporal pulse shape and its amplitude, in real time, would offer the possibility of controlling various complex photochemical processes and quantum control of interactions on molecular time scales. It is desirable to use self phase modulation of a seed beam to produce a train of bandwidth-limited short-duration output pulses and further, to produce a train of short-pulse, high repetition-rate, high energy output pulses.